1. Field of the Invention
The present invention relates to an amorphous soft magnetic alloy powder which can be produced by using a water atomization method, and a dust core and a wave absorber using the same.
2. Description of the Related Art
Conventionally, a Fe—Al—Ga—P—C—B—Si-based alloy is known as an amorphous soft magnetic alloy in which an amorphous phase can be formed by quenching a molten alloy (for example, refer to U.S. Pat. No. 5,738,733 or U.S. Pat. No. 5,876,519). Of the conventional amorphous soft magnetic alloys, some amorphous soft magnetic alloys having a specific composition are known as metal glassy alloys which have a wide temperature region in which they are in a state of a supercooled liquid before crystallization. It should be noticed that these metal glassy alloys have excellent soft magnetic characteristics and form easily bulky alloys having a thickness larger than the conventionally known amorphous alloy ribbon having a different composition, which is prepared by a liquid quenching method.
However, because these metal glassy alloys are produced by a liquid quenching method such as a single roll, it is required to improve their own amorphous phase-forming abilities to some extent. Therefore, the main object in the development of such a metal glassy alloy was to improve its amorphous phase-forming ability, and the development has progressed from investigations of an alloy composition capable of achieving this object. However, the composition which is capable of increasing the amorphous phase-forming ability of the alloy does not always coincide with the composition which is capable of increasing soft magnetic characteristics, and thus there is still room for further improvement in a high saturated magnetization and soft magnetic characteristics.
Further, since the metal glassy alloy having the conventional composition contains a high-priced gallium (Ga), it is not appropriate for the mass production. Therefore, the glassy alloy is desired to have a composition capable of decreasing the manufacturing cost.
On the other hand, the glassy alloy manufactured by the single roll method can be obtained as a ribbon having a thickness of about 200 μm. For applying this ribbon to a magnetic core such as a trans and a choke coil, the ribbon is grinded into a powder, the powder is mixed with a binder such as a resin, and the resultant mixture is solidified and molded to produce a dust core.
In order to overcome the above-mentioned problems, a soft magnetic alloy powder such as a Fe—Al—Si-based alloy and a Mo permalloy (for example, refer to U.S. Pat. No. 5,651,841) has been proposed. The method of producing this soft magnetic alloy powder has employed a gas atomization method in which a molten alloy is quenched by spraying an inert gas thereto, or a water atomization method in which a molten alloy is quenched by blowing the molten alloy into water.
When the Fe—Al—Si-based alloy powder is used, a relatively low core loss is obtained, but a saturated magnetization is low and a DC superimposing characteristic is deteriorated. Further, Mo permalloy has a high core loss, and thus there is room for improvement in the practical use thereof. Therefore, in order to solve such problems, there is an attempt for obtaining a dust core having characteristics of a high saturated magnetization and a low core loss by pulverizing a Fe-based amorphous soft alloy, but there are problems in that the optimization of the shape of the powder is not sufficiently made and it is difficult to obtain excellent magnetic characteristics in the dust core of the amorphous alloy powder.
According to a gas atomization method, it is possible to obtain an amorphous soft alloy powder which has a spherical shape and a small amount of impurity (the content of oxygen is small). However, since an expensive inert gas is used in a large quantity to grind and cool down a molten alloy, the manufacturing cost increases. Further, it is difficult to make a manufacturing apparatus large to grind the molten alloy by using an inert-gas jet. Furthermore, since the inert gas is supplied from a gas bomb, the grinding pressure is merely increased to about 20 MPa, and it was difficult to increase a manufacturing efficiency. Therefore, the amorphous soft magnetic alloy powder produced by the gas atomization method has a problem in that the manufacturing cost thereof is high and thus it is not suitable for the mass production thereof.
Therefore, it is studied and investigated to employ a water atomization method which is conducted under an atmosphere of air, instead of the gas atomization method. If the water atomization method is employed, it is possible to make the manufacturing apparatus large and the molten alloy can be jetted at a high pressure, and thus the mass production can be enhanced. Further, since the cooling velocity in the water atomization method is generally high as compared to the case in which the inert gas is used, it is easy to make the molten alloy amorphous. However, when the metal glassy alloy is made by using the water atomization method, there are problems in that liquid droplets of a high temperature molten alloy are quenched while coming into contact with water to easily corrode components of the alloy uselessly, and thus a large oxidized portion results in the obtained powder.
In view of such a background, the inventors of the present invention have developed a glassy alloy of the composition into which an element such as Cr and a noble metal is added for enhancing a corrosion-resistant effect, as a composition in which the corrosion hardly occurs even though the water atomization method is used, have tried to improving the characteristic of the glassy alloy powder, and have made progress the research and development in JP-A No. 2002-226956 or No. 2004-156134.